In view of (1) the increasing evidence that "killer" lymphocytes, capable of destroying tumor cells are present in all tumor hosts but are prevented from reaching their targets by "blocking" serum factors (i.e. tumor antigens or antigen-antibody complexes); (2) the discovery made recently in the applicant's laboratory that tumor bearing hosts possess "suppressor" T cells capable of counteracting in an immunologically specific manner the cytoxic activity of effector T lymphocytes of syngeneic animals immune to the tumor; (3) the complexity of the diverse cellular interactions underlying the immune system and the lack of definitive knowledge as to how to regulate it so as to tilt the immunological balance in favour of cell-mediated immunity (CMI), it is proposed to circumvent these difficulties by designing immunologically specific strategies for destruction of tumor cells with the aid of modified antibodies to tumor antigens (TA) to be used as (a) carriers of cytotoxic drugs or "homing" devices for attracting phagocytic cells and lymphocytes activated to a state of CMI into the microenvironment of malignant cells or foci. For this purpose, attempts will be made to produce pure anti-TA antibodies in sufficient amounts for further chemical manipulations involving (1) the conjugation of cytotoxic drugs by covalent bonds to these antibodies; (2) complexing of such drugs to these antibodies by noncovalent, but immunochemically specific bonds; (3) attachment of antigenic "markers" to these antibodies which, after fixation on tumor cells, will interact with lymphoid cells in a state of CMI to these cells. The presence of a fibrin matrix within many common tumor foci will be exploited for their destruction by similar mechanisms with the aid of antibodies (similarly modified) directed against the distinct antigens of fibrin rather than those of tumor cells (this approach had already yielded promising results in the applicant's laboratory). An attempt will also be made to isolate the soluble immunosuppressive factor released from suppressor T cells and to produce an antiserum to it.